This invention relates to an apparatus and method for the selection of light of one optical frequency or wavelength channel from a multiplicity of channels or optical frequencies being transmitted in an optical fiber or optical waveguide. The invention can also be used to efficiently combine light waves of different frequencies.
Filter technology for wavelength division multiplexing (WDM) is one of the most active research and development topics in the optical fiber telecommunications field. A number of guided wave filter approaches are at various stages of development, including the fiber Bragg grating, fiber Fabry-Perot, asymmetrical Mach Zehnder interferometer (MZI), waveguide grating router (WGR), acoustooptic tunable filter (AOTF), and electrooptic tunable filter (EOTF).
Of these technologies, only the AOTF and EOTF can be tuned from one channel to another at near-microsecond (for the AOTF) or sub-microsecond (for the EOTF) speeds needed for fast packet-switched networks. As illustrated in FIGS. 1 and 2, conventional, prior art, schemes for implementing the AOTF and EOTF have several features in common. Both are fabricated in ferroelectric insulating substrates such as lithium niobate, and make use of a four-port MZI structure formed from waveguides which are single mode for both horizontally polarized light (TE) and vertically polarized light (TM). Both filters make use of phase-matched polarization conversion in the arms of the MZI, and ideally their performance is independent of the incident polarization state. Another common feature of the conventional AOTF and EOTF designs is that they both use polarizing beam splitters (PBSs), optical circuit elements which have proven difficult to fabricate with the high polarization extinction ratios needed to meet filter requirements.
A prior art four-port beam splitter is illustrated in FIG. 3. Light incident in port 1 will, in general, be split between output port 1 (straight through port) and output port 2 (crossover port). The requirement for a PBS is that, for i,j=1,2: (fTE)ii=1; (fTE)ij=0, jxe2x89xa0i; (fTM)ii=0; (fTM)ij=1, jxe2x89xa0i, where (fP)ij is the fraction of the power in input port i which couples to output port j for polarization P. (It should be noted that an alternative PBS design is obtained by reversing xe2x80x9cTExe2x80x9d and xe2x80x9cTMxe2x80x9d in these expressions). As more fully disclosed hereafter, Applicants"" invention is directed to new AOTF and EOTF configurations which allow for an additional degree of freedom in beam splitter characteristics and which are, therefore, much easier to fabricate than the conventional filters.
The prior art AOTF depicted in FIG. 1 makes use of the strain-optic effect from a traveling acoustic wave to produce polarization conversion in the two arms of the MZI structure, which is fabricated on a LiNbO3 substrate. The conversion is very efficient at the optical frequency xcexdj for which a phase matching condition is satisfied, such that the acoustic wavelength exactly matches the TE-TM polarization beat length in the waveguide. At other optical frequencies for which the phase matching condition is not satisfied, little polarization conversion occurs. Horizontally polarized (TE) light incident on the filter at a frequency xcexdi is directed by the first PBS into its straight through output portxe2x80x94the upper waveguide in the Mach-Zehnder. If the polarization is not converted in that waveguide (ixe2x89xa0j), the light incident on the second PBS is also directed to its straight through output port, which is the upper output port of the filter. If the polarization is converted (i=j), the light incident on the second beam splitter emerges from its crossover port, which is the filter""s lower output port.
On the other hand, vertically polarized (TM) light incident on the filter at a frequency xcexdi is directed by the first PBS into its crossover output port, the lower waveguide in the MZI. If the polarization is not converted in that waveguide (ixe2x89xa0j), the light incident on the second PBS is also directed to its crossover port, the filter""s upper output port. If the polarization is converted (i=j), the light incident on the second beam splitter emerges from its straight through output port, which is the filter""s lower output port. Thus, for either polarization, TE or TM, the light at the selected frequency xcexdj emerges from the lower output port of the filter, and all other frequencies exit via the upper output port. Tuning of the filter to change the selected optical frequency is accomplished by changing the acoustic frequency.
Conceptually, the conventional EOTF differs from the AOTF in two respects: both the polarization coupling mechanism and the tuning method are different. In the EOTF illustrated in FIG. 2, tuning is accomplished by an applied voltage Vj which changes the waveguide birefringence and hence the optical frequency xcexdj for which phase matching occurs. A spatially periodic strain-inducing film causes polarization coupling via the strain-optic effect. In other EOTF designs, a spatially periodic electric field produced by an interdigital electrode structure induces the polarization coupling via the electrooptic effect.
Performance of the four-port beam splitter of FIG. 3 is described by the expression
OP=CPIP,xe2x80x83xe2x80x83(1) 
with                                           I            P                    =                      [                                                                                                      (                                              I                        P                                            )                                        1                                                                                                                                          (                                              I                        P                                            )                                        2                                                                        ]                          ,                            (        2        )                                                      O            P                    =                      [                                                                                                      (                                              O                        P                                            )                                        1                                                                                                                                          (                                              O                        P                                            )                                        2                                                                        ]                          ,                            (        3        )            
and                               C          P                =                  [                                                                                          (                                          c                      P                                        )                                    11                                                                                                  (                                          c                      P                                        )                                    12                                                                                                                          (                                          c                      P                                        )                                    12                                                                                                  (                                          c                      P                                        )                                    11                                                              ]                                    (        4        )            
In these expressions P represents the polarization (TE or TM), (IP)i is the input electric field amplitude in port i (i=1,2), (OP)i is the corresponding output electric field amplitude, and the coupling coefficients are
(cP)11=cos(xcexaPL)xe2x80x83xe2x80x83(5) 
(cP)12=isin(xcexaPL),xe2x80x83xe2x80x83(6) 
with xcexaP the interwaveguide coupling coefficient and L the effective length of the coupling region. The analysis neglects loss and assumes that the coupled waveguides are identical and support a single mode for each polarization, but that in general the mode field patterns and hence the coupling coefficients are different for the two polarizations.
For a polarizing beam splitter with TE polarization directed in the straight through path and TM polarization crossing over, the coupling coefficients must satisfy these conditions: (cTE)11=1, (cTE)12=0; (cTM)11=0; (cTM)12=1. For these relations to hold, xcexaTML=(2 m1xe2x88x921)xcfx80/2, xcexaTEL=m2xcfx80, with m1 and m2 positive integers. Thus, constraints on both coupling coefficients must be met simultaneously to satisfy the requirements for a PBS. Furthermore, from a practical standpoint it is desirable to make the coupler as short as possible, implying small values of m1 and m2. For m1=m2=1, for example, xcexaTML=xcfx80/2 and xcexaTEL=xcfx80,_so xcexaTE=2xcexaTM. This implies a considerably broader mode profile for the TE mode than for the TM mode, which is undesirable from the standpoint of mode matching to an optical fiber.
Accordingly, as opposed to those now known in the industry, the AOTF and EOTF apparatus and methods of the present invention do not require polarizing beam splitters. Further, because the invention provides an additional degree of freedom in achieving the required beam splitter performance, it is much easier to fabricate than prior art AOTFs and EOTFs which make use of polarizing beam splitters. In particular, a guided wave tunable filter of the present invention includes, in a preferred embodiment, two 3-port Y-branch beam splitters connected to form two spaced apart waveguides between said beam splitters, with an input port and an output port. The waveguides include an optical path difference of half a wavelength and polarization coupling regions in the two waveguides are displaced by half the spacial period of a pertubation responsible for coupling. Both acoustooptic and electrooptic tunable filters of the two port design are disclosed. Additionally, in a preferred embodiment, a four port AOTF and a four port EOTF are also disclosed.